Device for transmitting torque between two rotatable, coaxial schaft members

ABSTRACT

A device for transmitting torque between two rotatable, coaxial shaft members ( 1, 2 ) contains a clutch ( 35 ) between the two shaft members and engageable—to counteract rotational speed differential between the shaft members—by a hydraulic piston ( 18 ) under the control of an electrically controlled throttle or pressure valve ( 26 ), and a clutch pump ( 36 ) for supplying hydraulic pressure to the hydraulic piston, the clutch pump being driven by either one or both of the shaft members. The hydraulic piston ( 18 ), the valve ( 26 ), and the clutch pump ( 36 ) are connected in a hydraulic system also containing a feeder pump ( 31 ) for maintaining a certain base pressure in the system. The hydraulic system conditionally allows flow from the feeder pump ( 31 ) past the valve ( 26 ) to the hydraulic piston ( 18 ) irrespective of the flow from the clutch pump ( 36 ).

This application claims priority from pending Swedish Patent ApplicationNo. 0203618-4 filed Dec. 6, 2002.

FIELD OF THE INVENTION

The present invention relates to a device for transmitting torquebetween two rotatable, coaxial shaft members, the device containing

a clutch between the two shaft members and engageable—to counteractrotational speed differential between the shaft members—by means of ahydraulic piston under the control of an electrically controlledthrottle or pressure valve,

and a clutch pump for supplying hydraulic pressure to the hydraulicpiston, the clutch pump being driven by either one or both of the shaftmembers,

wherein the hydraulic piston, the valve, and the clutch pump areconnected in a hydraulic system also containing a feeder pump formaintaining a certain base pressure in the system.

The invention especially relates to such a device, wherein the clutchpump is a differential pump driven by the rotational speed differentialbetween the two shaft members. Reference is below only made to such adifferential pump.

BACKGROUND OF THE INVENTION

A practical device of this kind is used as a so called “limited slipcoupling” in a four-wheel-driven car for transmitting force for examplefrom the driving forward axle of the car to its driven rear axle. Insuch a case, the coupling is arranged in the longitudinal drive shaftbetween the forward and rear axles.

The coupling is based on the principle that when there is a rotationalspeed differential between the two shaft members in the coupling, thedifferential pump will create a hydraulic flow which—moderated by theelectrically controlled valve—will be transmitted to the clutch forcounteracting the rotational speed differential.

A typical example of such a limited slip coupling is shown in WO97/04245, to which reference is made for further information.

A possible drawback with the coupling shown therein and referred toabove is that it may in certain cases be desirable to transmit a certainmoment already when the vehicle or car stands still—a so called “lockupfrom stationary”. As the design so far described is dependent for itsfunction on a certain rotational speed differential this is notpossible.

The main object of the invention is thus to modify the device orcoupling as so far described to enable a “lockup from stationary”.

SUMMARY OF THE INVENTION

This may according to the invention be attained by means in thehydraulic system for conditionally allowing flow from the feeder pumppast the valve to the hydraulic piston irrespective of the flow from thedifferential pump.

The inventive idea is accordingly to make use of the flow from thefeeder pump, which is available for the purpose of maintaining a certainbase pressure in the hydraulic system required for the ordinary functionof the device.

In a practical case—as is shown in WO 97/04245—the hydraulic system—in aclosed portion—comprises lines from the differential pump, through theelectrically controlled throttle or pressure valve to the clutch, theportion including check-valves. However, oil may be supplied by thefeeder pump from a reservoir and may be removed or returned to thereservoir via an overflow valve.

In such a system the feeder pump and the differential pump may inaccordance with the invention either be connected in series or inparallel in order to fulfill the purpose of conditionally (i.e. throughthe electrically controlled throttle or pressure valve) using the feederpump flow for actuating the clutch.

In an arrangement for series connection a separate check-valve isarranged between the overflow valve and the feeder pump.

Here, an accumulator may be arranged either in connection with theoverflow valve or the feeder pump.

In an arrangement for parallel connection a separate check-valve isarranged between the feeder pump and the electrically controlledthrottle or pressure valve.

In order to prevent return flow, when the fluid pressure exceeds that ofthe feeder pump, a separate overflow valve may be connected in parallelover the check-valve in all the embodiments described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in further detail below under referenceto the accompanying drawings, in which

FIG. 1 is a sectional view of a prior art limited slip coupling with afirst embodiment of a hydraulic system according to the invention,

FIGS. 2–4 diagrammatically show other embodiments of hydraulic systemsaccording to the invention, and

FIGS. 5–10 diagrammatically and in a different fashion show thedifferent embodiments of a hydraulic system shown in FIGS. 1–4.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention is concerned with a hydraulic system for a socalled limited slip coupling of the type usually used to transmit torquefrom a first driven axle of a four-wheel drive road vehicle to a seconddriven axle, or differently speaking for transmitting torque between tworotatable, coaxial shaft members.

Such a limited slip coupling is for example known through WO 97/04245,to which reference is made. The design of this coupling is also shownand described here for providing a proper understanding of the hydraulicsystem according to the invention. This description is kept as short aspossible.

A first shaft 1 to the left in FIG. 1 and a second shaft 2 to the rightare rotatably journalled in a housing 3 with a cover 4.

A hub 5 is attached to the first shaft 1 and constitutes a first shaftmember therewith.

A number of alternate clutch discs or rings 6 are externally splined tothe hub 5.

The second shaft 2 has a radial flange 2′, and a sleeve 7 is attachedthereto. The shaft 2 with its flange 2′ and the sleeve 7 togetherconstitute a second shaft member.

A number of alternate clutch discs or rings 8 corresponding to theclutch discs 6 are internally splined to the sleeve 7. When engaged, theclutch discs 6 and 8 will counteract rotational speed differentialbetween the two shaft members.

A cam disc 9 is splined to the hub 5 to the right of the clutch discs 6,8 in the drawing for their engagement in a way to be described, and thereaction force is taken up by the housing 3 via a thrust washer 10,which is positioned to the left of the clutch discs 6, 8 in the drawing,and an axial needle bearing 11, supported by the housing 3.

At its surface facing from the clutch discs 6, 8 the cam disc 9 isprovided with a circular cam curve, and the axial end surface of the hub5 is provided with a corresponding circular cam curve. Three sets ofcylindrical cam rollers 12–14 are arranged to cooperate with these camcurves. The cam rollers 12–14 are rotatably arranged in the radialflange 2′ of the second shaft 2. Each set may contain for example threecam rollers, and there is a phase shift from one set to the next one,for example amounting to 30°. The cam profile on the cam disc 9 is suchthat at rotation the cam rollers 12–14 will be imparted a movement whoseaxial speed is linearly changed with the relative rotational anglebetween the shafts 1 and 2.

Thrust washers 15–17 are in engagement with the cam rollers 12–14. Thesethrust washers 15–17 are concentrical and rotatable independently ofeach other.

Three concentrical, ring-shaped pistons 18–20 are axially movable incorresponding ring–shaped cylinders in the housing cover 4 and areprovided with sealings. The two inner pistons 19, 20 cooperating withthe hub 5 have the same piston area, whereas the outermost piston 18cooperating with the cam disc 9 has a greater area.

The result of the described arrangement is that the pistons 18–20 willbe imparted axially reciprocating movements, when the two shafts 1 and 2have a rotational speed differential.

A hydraulic line 21 provided with a check valve 22 is connected to eachof the two ring-shaped cylinders for the inner pistons 19, 20. After thecheck valves 22 the two lines 21 are connected to each other and lead toan overflow line 23 (here shown to be provided with a check valve 24 tobe described below in connection with the invention), and further to ahydraulic line 25 provided with an electrically controlled throttlevalve 26. The hydraulic line 25 is connected to the ring-shaped cylinderfor the outermost piston 18 by a hydraulic line 27. This latter line 27is connected to the hydraulic lines 21 via check valves 28. Thishydraulic system is a closed system.

The hydraulic line 23 is also connected to an oil reservoir 29 via anoverflow valve 30, for example adjusted to a pressure of 3 bar. Oil fromthe reservoir 29 is pumped by means of a motor-driven pump 31 to thehydraulic lines 21. The main purpose of the pump 31, which below iscalled a feeder pump, is to establish a certain working pressure in theclosed hydraulic system. The system will not be able to function asintended without this system pressure.

An accumulator 32 is connected to the hydraulic system (at the hydraulicline 23) for leveling out variations in the flow to the pistons 18–20.

The purpose of the electrically controlled throttle valve 26 is tocontrol the working pressure in the closed hydraulic system and thus tocontrol the force with which the rotational speed differential will becounteracted.

For the sake of clarity, the hydraulic system is shown as being arrangedoutside the housing 3, 4. In reality it is arranged within the housing,which in itself functions as the reservoir 29.

When the described hydraulic system is under pressure, so that thepistons 18–20 are held against the thrust washers 15–17, a rotationalspeed differential between the two shafts 1 and 2 will effect areciprocal movement of the pistons 18–20, as the cam rollers 12–14 rollon their respective cam curves. The frequency of the reciprocalmovements increases with increasing rotational speed differential.

For a further description of the device and its function reference ismade to WO 97/04245.

The main constituents of the device are

a “differential pump”, namely the hydraulic pump creating a hydraulicflow depending on the rotational speed differential between the twoshafts,

a “clutch”, namely the hydraulic piston for actuating the clutch discsand accomplishing a torque transmission between the two shafts, when theflow passes the electrically controlled throttle valve, and

a “feeder pump” with an overflow valve for maintaining a certain basepressure in the otherwise closed system.

A certain rotational speed differential is needed for building up thehydraulic pressure required for actuating the clutch. This will preventa so called “lockup from stationary”, i e an actuation of the clutch forenabling the transmission of torque between the two shafts, when thevehicle (in which the device is arranged) is stationary.

The present invention is aimed at enabling a “lockup from stationary” ina device of the general kind described above.

Generally speaking, this may be accomplished by utilizing the feederpump for supplying the necessary hydraulic pressure to the piston foractuating the clutch, even if the differential pump can not deliver anypressure, as the vehicle is stationary.

In a first embodiment as shown in FIG. 1 this is accomplished by theprovision of the above-mentioned check valve 24 in the hydraulic line 23and of a hydraulic line 33, which contains an overflow valve 34 and isparallel to the hydraulic line containing the throttle valve 26.

This first embodiment is also illustrated by the hydraulic scheme inFIG. 5.

The reference numeral 35 is used for the “clutch”, namely tocollectively indicate the hydraulic piston 18 actuating the clutch discs6 and 8 connecting the two shafts 1 and 2. The reference numeral 36refers to the “differential pump” including the pistons 19 and 20 andadjoining members and the check valves 22 and 28.

Also shown in FIG. 5 are the check valve 24, the throttle valve 26(which alternatively could be an electrically controllable pressurevalve), the reservoir 29, the overflow valve (or relief valve) 30,typically set at a pressure of 3–5 bar, the feeder pump 31, and theaccumulator 32.

The feeder pump 31 establishes a hydraulic pressure in the accumulator32 (through the throttle valve 26) and will provide a increased pressurein the clutch, when the rotational speed differential is nil and thethrottle valve 26 is partly or fully closed.

It appears that the feeder pump 31 and the differential pump 36 areconnected in series before the throttle valve 26. The check valve 24 issituated between on one hand the feeder pump 31 and on the other handthe accumulator 32 and the overflow valve 30.

In FIGS. 2–10 only reference numerals needed for a proper understandingare used. All other numerals appear in FIG. 1.

A second embodiment is shown in FIGS. 2 and 6. The only difference inrelation to the first embodiment is the position of the accumulator 32,which here is connected to the hydraulic line 21 to the feeder pump 31.

This means that the feeder pump 31 and the differential pump 36 areagain connected in series, but that the feeder pump 31 and theaccumulator 32 are at the same side of the throttle valve 26.

A third embodiment is shown in FIGS. 3 and 7. The difference in relationto the first embodiment is that a further overflow valve 37 is connectedin parallel with the check valve 24. Hereby the maximum pressure fromthe feeder pump 31 will be limited.

A fourth embodiment is shown in FIG. 8 (and FIG. 4, as will appear). Thebasic thought behind the fourth embodiment is to arrange the feeder pump31 and the differential pump 36 in parallel before the throttle valve26. A check valve 38 in the hydraulic line from the feeder pump 31 willprevent back-flow to the feeder pump 31, if the differential pump 36 is“stronger”.

A variation, being in effect a combination of the third and fourthembodiments, is shown in FIG. 9, where the overflow valve has receivedthe numeral 37′.

A detailed hydraulic scheme for a combination of the third and fourthembodiments is also shown in FIG. 4.

A fifth embodiment is shown in FIG. 10, which is illustrated as avariation of FIG. 5 (the first embodiment). In this embodiment thethrottle valve 26 is combined with a pressure transducer 39. By theprovision of this pressure transducer 39 it will be possible to lock theclutch at a certain pressure at the rotational speed differential nil,then to intermittently operate the feeder pump at high power andthereafter to decrease the power for i.e. a saving the pump motor. Thisembodiment can be combined with all the other embodiments.

1. A device for transmitting torque between two rotatable, coaxial shaftmembers, the device containing a clutch between the two shaft membersand engageable to counteract rotational speed differential between theshaft members by means of a hydraulic piston under the control of anelectrically controlled throttle or pressure valve, and a clutch pumpfor supplying hydraulic pressure to the hydraulic piston, wherein theclutch pump is a differential pump driven by the rotational speeddifferential between the two shaft members, wherein the hydraulicpiston, the valve, and the clutch pump are connected in a hydraulicsystem also containing a feeder pump for maintaining a certain basepressure in the system, characterized by means in the hydraulic systemfor conditionally allowing flow from the feeder pump past the valve andto the hydraulic piston to actuate the clutch irrespective of the flowfrom the clutch pump.
 2. A device according to claim 1, in which thehydraulic system in a closed portion comprises lines from thedifferential pump, through the electrically controlled throttle orpressure valve to the clutch, the portion including check-valves, and inwhich oil may be supplied by the feeder pump from a reservoir and may beremoved to the reservoir via an overflow valve, wherein the feeder pumpand the differential pump are connected in series by means of acheck-valve between the overflow valve and the feeder pump.
 3. A deviceaccording to claim 2, wherein an accumulator is arranged in connectionwith the feeder pump.
 4. A device according to claim 2, wherein anaccumulator is arranged in connection with the overflow valve.
 5. Adevice according to claim 1, wherein the feeder pump and thedifferential pump are connected in parallel by means of a check-valvebetween the feeder pump and the electrically controlled throttle orpressure valve.
 6. A device according to claim 5, wherein an overflowvalve is connected in parallel over the check-valve.
 7. A deviceaccording to claim 6, wherein a pressure transducer is arranged inconjunction with the electrically controlled throttle or pressure valve.